var TEST = function () {  
    
    let _oriStr;
    let _revStr;
    let _ti;
    
    function _isMatch(str, si, ei) {
        return _oriStr.substring(si, ei+1) === _revStr.substring(_ti-ei, _ti-si+1);
    }
    
    return {
        doTest: function (str) {
            if (!str) { return ''; }
			
            let len = str.length;
            _oriStr = str;
            _revStr = str.split('').reverse().join('');
            _ti = len - 1;
			
            const idx = { s: NaN, e: NaN };
			while (len > 0) {
                idx.s = 0;
                idx.e = len - 1;
				while (_oriStr[idx.e]) {
					if (_isMatch(_oriStr, idx.s, idx.e)) {
						return _oriStr.substring(idx.s, idx.e+1);
					}
					idx.s++;
					idx.e++;
				}
				len--;
			}
            return _oriStr[0];
        }
    };
}();